1. Field of the Invention
The present invention relates to a method and a device for analysing an optical wavefront. It represents an improvement of the methods of wavefront analysis based on local measurement of the slope of the wavefront.
2. Description of the Related Arts
Analysis of a wavefront by local measurement of the slope (corresponding to the local derivative of the phase of the wavefront) is for example the principle of wavefront analysers called xe2x80x9cShack-Hartmann arrayxe2x80x9d wavefront analysers. Generally they have an array of spherical microlenses and a array detector, each microlens focusing the surface element of the wavefront intercepted by the subaperture corresponding to the microlens, thus forming a light spot on the detector. The local slope of the surface element is determined from the position of the spot on the detector. Actual analysis of the wavefront surface, i.e. reconstruction of the phase of the wavefront for example on a base of polynomials, can be obtained by integration of the local measurements of the slope. Other types of analysers work on a line of the wavefront. Cylindrical microlenses arranged linearly and a detector with linear geometry are, for example, used in this case. In the same way as in the Shack-Hartmann array type, the local slopes of the wave line are measured from the positions of the spots formed by the microlenses.
Generally, the method according to the invention applies to any type of wavefront analysers based on measuring the local slope of the wavefront. The term xe2x80x9carray of microlensesxe2x80x9d will be used hereinafter for any set of microlenses for use in this type of analyser, it being possible to arrange the microlenses linearly or according to a two-dimensional array. Similarly, we shall talk of analysis of a xe2x80x9cwavefrontxe2x80x9d, and this analysis can relate equally to a part of the surface of the wavefront, in particular a line of the wavefront or the complete surface of the wavefront.
FIG. 1 shows an assembly ML of microlenses Li and a detector DET for implementing a method of wavefront analysis as described above. When a wavefront F1 enters the system, each microlens forms a spot Ti on the detector. To determine the position of the spots, generally it is assumed that a spot Ti formed by a given microlens Li is within an assumed localization zone Zi. This localization zone is for example defined by the projection on detector DET of the subaperture SPi corresponding to the microlens Li, as shown in FIG. 1. This assumption offers the advantage of considerably simplifying the circuit for localization of the spots, thus making the system faster. Sometimes the structure of the array of microlenses is not perfect and may have local defects, for example defects in arrangement of the microlenses or defects relating to the size of one microlens relative to another. This introduces an error in the position of the spot formed. To overcome this type of problem, generally the positions of the spots formed from a reference beam that is known perfectly are subtracted from the positions of the spots formed from the wavefront to be analysed. Of course, to avoid introducing any error during this operation, it is necessary for the positions of the two spots formed by the same microlens to be subtracted from one another. If it is assumed a priori that a spot detected in a given localization zone has come from the subaperture that defines this zone, there is a risk of introducing an error during the subtraction operation when a wavefront has a considerable deflection, for example. Thus, as can be seen for example in FIG. 1, if a wavefront F2 has considerable deflection, the spot Ti formed by lens Li is within the assumed localization zone Zi+1 corresponding to lens Li+1. There is a displacement of a subaperture (in the chosen example) between the subaperture SPi from which spot Ti originates and the subaperture Spi+1 defining the localization zone Zi+1 in which the spot Ti is actually located.
Of course, we always try to obtain perfect arrays of microlenses and the technology is advancing in this direction. However, the problem of knowing with certainty the correspondence between a detected spot and the subaperture from which it originated always arises, for example when we require exact measurement of the deflection using a device that is required to have a wide dynamic range, i.e. a device capable of analysing wavefronts possessing large deflections, among other things. In this case, to know this correspondence with certainty, it is necessary to be able to measure the displacement between the subaperture from which the spot originated and the subaperture that defines the assumed localization zone in which the spot is located.
A solution has been proposed in this direction by the company Adaptive Optics Associates (AOA, Cambridge, Mass.). This solution, applied to a wavefront analyser of the Shack-Hartmann array type, is explained in the article xe2x80x9cHartmann sensors detect optical fabrication errorsxe2x80x9d (LASER FOCUS WORLD, April 1996). It consists, in the course of measurement, of bringing the detector of the array of microlenses closer, in such a way that, regardless of the local slope of the wavefront being analysed, all the flux collected by a subaperture is located totally within the assumed localization zone defined by this subaperture. Then the detector is moved farther away from the array of microlenses as far as its normal working position while following the position of the spot. It is thus possible to detect whether it changes zone. This solution has some drawbacks. In particular, it necessitates movement of the detector, which involves mechanical constraints in the system and the risk of introducing an error in the measurement, because of possible deflection of the detector, or poor axial repositioning during movement. Furthermore, this calibration operation must be repeated for each analysis of a new wavefront. And even in the course of analysis of a wavefront, since the correspondence between a spot and the microlens from which it originated is determined by following the position of this spot, if this position is lost (for example because the flux is cut off momentarily), the correspondence is no longer certain and recalibration becomes necessary.
To overcome these drawbacks, the present invention proposes another solution permitting exact measurement of the parameters of the wavefront and in particular of its deflection. It consists of choosing an array of microlenses having one or more local variations of its structure. According to one example of implementation, each local variation can be a difference in positioning of one or of several microlenses. This variation can be an unwanted defect of the array or a local variation introduced in a controlled manner during manufacture. Comparing the positions of the spots formed starting from a wavefront to be analysed with the positions of the spots formed for example starting from a known reference wavefront, it is possible, owing to the presence of the local variation of structure which for example introduces variations in the positions of certain spots, to measure any displacement between the subaperture from which a detected spot originated and the subaperture that defines the assumed localization zone in which the spot is located.
More specifically, the invention relates to a method of wavefront analysis based on local measurement of the slope of the wavefront, the method comprising a stage of wavefront acquisition consisting of:
a stage of detection of the wavefront especially by means of an array of microlenses, a detector and means for processing the signal; each microlens defines an indexed subaperture, and focuses a surface element of the wavefront, intercepted by the said subaperture; a spot is formed on the detector which delivers a signal; an assumed localization zone of the spot on the detector is defined for each subaperture.
a stage of processing of the signal supplied by the detector, permitting establishment of a measurement file; this file associates in particular with each subaperture in the localization zone from which a spot is detected, the position of this spot, each subaperture being referenced by its index.
The method is characterized in that it further comprises:
prior choice of an array of microlenses having at least one local variation of its structure,
a preliminary stage of characterization of this array making it possible to establish a reference file associating in particular with each subaperture, referenced by its index, the position of the spot that originated from the said subaperture when the subaperture is illuminated by a known wavefront, the file data including a contribution due to the local variation of the structure of the array,
during each wavefront analysis,
establishment of the measurement file, the file data also including a contribution due to the local variation of the structure of the array,
comparison of the said contributions taken from each of the two files, this comparison making it possible to determine any displacement in number of subapertures between these two contributions and to deduce therefrom with certainty the correspondence between a detected spot and the subaperture from which it originated,
knowing this correspondence, and on the basis of the measurement file and the reference file, calculating the average slope of the wavefront on each surface element intercepted by each subaperture illuminated by the wavefront.